1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer for receiving image data from, e.g., an external apparatus, and for printing received data.
2. Description of the Related Art
Image formation in an image forming apparatus such as a laser printer is attained via, e.g., electrification, exposure, developing, copying, peeling, cleaning, and electrification unit, an exposure unit, a developing it, a copy unit, a peeling unit, a cleaning unit, and the like are sequentially arranged around a photosensitive drum. The apparatus also comprises a fixing unit for receiving a paper sheet from the peeling unit. These units are driven upon rotation of the photosensitive drum, thereby executing the above-mentioned image formation processes. In this manner, an image can be formed on a paper sheet.
In a laser printer of this type, its control system is divided into an engine control section for controlling the above-mentioned units to control operations for attaining the image formation processes, and a printer control section for controlling operations of the engine control section. The engine and printer control sections are connected for exchanging commands and status data. The printer control section expands image information in units of pages supplied from an external equipment onto a partial memory to generate bit-image data, and then outputs the bit-image data to the engine control section.
Such a laser printer as mentioned above is provided with a ROM containing basic font patterns, and has a so-called "derive function." For the derive function, the buffer area (RAM) for expanding the bit image previously stores thick fonts, italic fonts, and modified fonts in which the direction of the font is rotated. These various fonts are derived from the basic fonts in the ROM. Since these derived fonts occupies a certain part of the RAM, the amount of the buffer area for expanding the bit image is reduced.
Data expanded in the buffer area of bit image expanding is transferred to the engine controller, and the corresponding image is formed therein. Because of the previous storing of the derived font data, if the buffer area of bit image expanding has been reduced to less than one required to fully hold the image data of one page, this image data has to be divided into pieces so that each data piece can be held in the reduced buffer area.
In this case, a number of expanding and transferring operations for the image data pieces ar repeated so as to complete a full image formation of the bit image data. Such repeated operations require a relatively long period of time. If these repeated operations were not completed within a predetermined time limit, the resultant image formation would be defective.
Even if font data produced by the derive function is not used in the bit image expanding page, defective expansion of the bit image could result when the bit image expanding area is actually narrowed by the produced font data.